The invention generally relates to powder coating application systems, and more particularly to spray guns that fluidize and deliver powder coatings to a work piece, using electrostatically charged powder.
Electrostatic powder spray guns are known for use in applying protective coatings to industrial and commercial products. Typically, a finely divided powder is transported through a spray head while entrained in an air or gaseous stream that is discharged from the spray head. The entrained powder is directed by the spray head toward a target article or work piece and is drawn toward the article or work piece by opposite electrostatic charges (at least effectively) on the powder and on the work piece. After the work piece has been coated with the charged powder, the article is heated. The powder melts and flows together to bond and form a more permanently adhered coating as it cools.
Electrostatic powder spray guns as described are well known in the art. Examples are disclosed in U.S. Pat. No. 4,079,894xe2x80x94Harjar, et al.; U.S. Pat. No. 4,143,819xe2x80x94Hastings; U.S. Pat. No. 4,380,320xe2x80x94Hollstein, et al.; U.S. Pat. No. 4,529,131xe2x80x94Rutz; U.S. Pat. No 4,543,274xe2x80x94Mulder; U.S. Pat. No. 4,580,727xe2x80x94Moos; U.S. Pat. No. 4,630,777xe2x80x94Hollstein, et al.; U.S. Pat. No. 4,706,890xe2x80x94Talacko; U.S. Pat. No. 4,747,546xe2x80x94Talacko; U.S. Pat. No. 4,886,215xe2x80x94Ruud; U.S. Pat. No. 5,056,720xe2x80x94Crum, et al.; U.S. Pat. No. 5,395,046xe2x80x94Knobbe, et al.; U.S. Pat. No. 5,402,940xe2x80x94Haller, et al.; and U.S. Pat. No. 5,622,313xe2x80x94Lader, et al. These patents teach various apparatus, methods, and systems for the electrostatic application of powder coatings to a work piece.
Conventional electrostatic powder coating apparatus typically comprise a spray gun that charges the powder in one of two ways. In one type of device, the gun has a high voltage charging electrode which produces a corona that charges the powder as powder particles move through the corona. Voltages in the range from about 30 to about 100 kilovolts (kV) are typically applied to the electrode in this type of spray gun. Gas propellant pressures in the range of 30 to 70 pounds per square inch (psi) or more are often required as well. While such guns are suitable for many industrial applications, they can be difficult and/or expensive to operate. Additionally, the high voltages and gas pressures required for satisfactory operation make these devices potentially dangerous, particularly for casual users such as small shops and home hobbyists.
In a second type of known device, the gun charges the powder by friction, i.e., triboelectrically, the electrostatic effect produced by rubbing a nonconductive article such as an inflated rubber balloon against certain fabrics. In triboelectric electrostatic spray guns, contact surfaces along the discharge path of the spray gun are constructed from an electrically insulating material, typically a polymer. During spraying, the finely divided powder particles are caused to impact the contact surfaces numerous times and thereby become frictionally charged. High voltage is not required, but propellant gas pressures in the range of 30 to 70 psi or more are often required to produce the necessary frictional contact to effectively charge the sprayed powder.
An undesirable consequence of the triboelectric interaction between the powder and the contact surfaces of the gun is the creation of a charge on the contact surfaces that is opposite in polarity to that imparted to the powder particles. This effect reduces the efficiency of the gun, and has resulted in various attempts in the art to control or reduce the build up of charge on the gun""s contact surfaces.
In U.S. Pat. No. 4,706,890xe2x80x94Talacko, a spray gun is disclosed which contains a discharge electrode disposed entirely outside the region where the powder flows and in electrical communication with the gas flowing in a gas conduit. The powder is accelerated, by pressurized gas, through an annularly shaped and axially extending channel in which the coating material is electrically charged by friction. The channel is defined between inner and outer members formed from different electrically insulating materials. Unwanted charge that builds up on the inner and outer members is discharged to the electrode through the flowing gas.
In U.S. Pat. No. 5,622,313xe2x80x94Lader, et al., a triboelectric powder spray gun is disclosed in which a charge is imparted triboelectrically to the powder by repeated impacts of the powder with internal contact surfaces formed from electrically insulating materials such as PTFE or polyamide. Lader et al., increase the charging effectiveness of their gun by using an electrode disposed within the gun barrel to produce a corona treatment of the contact surfaces to discharge the contact surfaces and eliminate the need for adjacent ground pathways within the powder flow path.
In U.S. Pat. Nos. 5,850,976 and 6,003,779, both issued to Robidoux, a spray gun for applying powder to coat a work piece uses triboelectric the properties in a plurality of tubes to charge powder particles electrostatically. The gun communicates with a source of pressurized air through a handle with an air valve controlling flow of the air. Powder in a container in fluid communication with the air flow is fluidized and entrained when the valve is opened. A nozzle with an internal surface defining a passageway to an open end communicates with the powder container. The internal surface of the nozzle frictionally imparts a net electrical charge of a first polarity to part of the powder by frictional contact during spraying. A plurality of discrete tubes are disposed in the central passageway and impart a net electrical charge of a second polarity to another part of the powder due to frictional contact as that part flows around and through the tubes. The powder then exits the nozzle and contacts the work piece. In U.S. Pat. No. 6,003,779, a perforated disk is disposed at the open end of the nozzle to provide for an even dispersal of powder on the target object to be coated. U.S. Pat. Nos. 5,850,976 and 6,003,779, are incorporated herein by reference.
The present invention provides a gun for spraying a powder onto a work piece to form a coating. In one embodiment the gun comprises a pistol-grip housing including an air valve coupled to a source of pressurized air. A powder container is disposed in fluid communication with the air valve so that powder within the container is fluidized by air from the source of pressurized air when the valve is opened. A nozzle is positioned within the housing, and includes a central passageway having an open end and which is disposed in fluid communication with the powder container. The central passageway frictionally imparts a net electrical charge of a first polarity to a first portion of the powder when the first portion of the powder contacts the surface during the spraying. A discrete member is disposed within the central passageway and is capable of imparting a net electrical charge of a second polarity to a second portion of the powder when the second portion of the powder contacts the tubes during the spraying.
In an alternative embodiment, a gun for spraying a powder onto a work piece to form a coating is provided that includes a pistol-grip housing having an air valve coupled to a source of pressurized air. The housing also supports a source of variable electrical potential. A powder container is disposed in fluid communication with the air valve so that powder within the container may be fluidized by air from the source of pressurized air when the valve is opened. A nozzle is positioned within the housing and includes a central passageway having an open end and disposed in fluid communication with the powder container. The central passageway frictionally imparts a net electrical charge of a first polarity to a first portion of the powder when the first portion of the powder contacts the surface during the spraying. A discrete member is positioned within the central passageway and is capable of imparting a net electrical charge of a second polarity to a second portion of the powder when the second portion of the powder contacts the tubes during the spraying. An emitter rod is positioned within the central passageway and comprises a proximal end and a distal end, where the proximal end is interconnected to the source of variable electrical potential.